Dose volume histogram analysis of lung radiation from chest wall treatment: comparison of electron arc and tangential photon beam techniques

A dosimetric and radiobiological evaluation of VMAT following mastectomy for patients with left-sided breast cancer

Background

To compare the dosimetric, normal tissue complication probability (NTCP), secondary cancer complication probabilities (SCCP), and excess absolute risk (EAR) differences of volumetric modulated arc therapy (VMAT) and intensity-modulated radiation therapy (IMRT) for left-sided breast cancer after mastectomy.

Methods and materials

Thirty patients with left-sided breast cancer treated with post-mastectomy radiation therapy (PMRT) were randomly enrolled in this study. Both IMRT and VMAT treatment plans were created for each patient. Planning target volume (PTV) doses for the chest wall and internal mammary nodes, PTV1, and PTV of the supraclavicular nodes, PTV2, of 50 Gy were prescribed in 25 fractions. The plans were evaluated based on PTV1 and PTV2 coverage, homogeneity index (HI), conformity index, conformity number (CN), dose to organs at risk, NTCP, SCCP, EAR, number of monitors units, and beam delivery time.

Results

VMAT resulted in more homogeneous chest wall coverage than did IMRT. The percent volume of PTV1 that received the prescribed dose of VMRT and IMRT was 95.9 ± 1.2% and 94.5 ± 1.6%, respectively (p < 0.001). The HI was 0.11 ± 0.01 for VMAT and 0.12 ± 0.02 for IMRT, respectively (p = 0.001). The VMAT plan had better conformity (CN: 0.84 ± 0.02 vs. 0.78 ± 0.04, p < 0.001) in PTV compared with IMRT. As opposed to IMRT plans, VMAT delivered a lower mean dose to the ipsilateral lung (11.5 Gy vs 12.6 Gy) and heart (5.2 Gy vs 6.0 Gy) and significantly reduced the V₅, V₁₀, V_20, V_30, and V₄₀ of the ipsilateral lung and heart; only the differences in V₅ of the ipsilateral lung did not reach statistical significance (p = 0.409). Although the volume of the ipsilateral lung and heart encompassed by the 2.5 Gy isodose line (V_2.5) was increased by 6.7% and 7.7% (p < 0.001, p = 0.002), the NTCP was decreased by 0.8% and 0.6%, and SCCP and EAR were decreased by 1.9% and 0.1% for the ipsilateral lung. No significant differences were observed in the contralateral lung/breast V_2.5, V_5, V₁₀, V₂₀, mean dose, SCCP, and EAR. Finally, VMAT reduced the number of monitor units by 31.5% and the treatment time by 71.4%, as compared with IMRT.

Conclusions

Compared with IMRT, VMAT is the optimal technique for PMRT patients with left-sided breast cancer due to better target coverage, a lower dose delivered, NTCP, SCCP, and EAR to the ipsilateral lung and heart, similar doses delivered to the contralateral lung and breast, fewer monitor units and a shorter delivery time.

Dosimetric verification of gated delivery of electron beams using a 2D ion chamber array

The purpose of this study was to compare the dosimetric characteristics; such as beam output, symmetry and flatness between gated and non-gated electron beams. Dosimetric verification of gated delivery was carried for all electron beams available on Varian CL 2100CD medical linear accelerator. Measurements were conducted for three dose rates (100 MU/min, 300 MU/min and 600 MU/min) and two respiratory motions (breathing period of 4s and 8s). Real-time position management (RPM) system was used for the gated deliveries. Flatness and symmetry values were measured using Imatrixx 2D ion chamber array device and the beam output was measured using plane parallel ion chamber. These detector systems were placed over QUASAR motion platform which was programmed to simulate the respiratory motion of target. The dosimetric characteristics of gated deliveries were compared with non-gated deliveries. The flatness and symmetry of all the evaluated electron energies did not differ by more than 0.7 % with respect to corresponding non-gated deliveries. The beam output variation of gated electron beam was less than 0.6 % for all electron energies except for 16 MeV (1.4 %). Based on the results of this study, it can be concluded that Varian CL2100 CD is well suitable for gated delivery of non-dynamic electron beams.

Monte Carlo calculation of monitor unit for electron arc therapy

PURPOSE

Monitor unit (MU) calculations for electron are therapy were carried out using Monte Carlo simulations and verified by measurements. Variations in the dwell factor (DF), source-to-surface distance (SSD), and treatment are angle (a) were studied. Moreover, the possibility of measuring the DF, which requires gantry rotation, using a solid water rectangular, instead of cylindrical, phantom was investigated.

METHODS

A phase space file based on the 9 MeV electron beam with rectangular cutout (physical size = 2.6 x 21 cm2) attached to the block tray holder of a Varian 21 EX linear accelerator (linac) was generated using the EGSnrc-based Monte Carlo code and verified by measurement. The relative output factor (ROF), SSD offset, and DF, needed in the MU calculation, were determined using measurements and Monte Carlo simulations. An ionization chamber, a radiographic film, a solid water rectangular phantom, and a cylindrical phantom made of polystyrene were used in dosimetry measurements.

RESULTS

Percentage deviations of ROF, SSD offset, and DF between measured and Monte Carlo results were 1.2%, 0.18%, and 1.5%, respectively. It was found that the DF decreased with an increase in a, and such a decrease in DF was more significant in the a range of 0 degrees-60 degrees than 60 degrees-120 degrees. Moreover, for a fixed a, the DF increased with an increase in SSD. Comparing the DF determined using the rectangular and cylindrical phantom through measurements and Monte Carlo simulations, it was found that the DF determined by the rectangular phantom agreed well with that by the cylindrical one within +/- 1.2%. It shows that a simple setup of a solid water rectangular phantom was sufficient to replace the cylindrical phantom using our specific cutout to determine the DF associated with the electron arc.

CONCLUSIONS

By verifying using dosimetry measurements, Monte Carlo simulations proved to be an alternative way to perform MU calculations effectively for electron are therapy. Since Monte Carlo simulations can generate a precalculated database of ROF, SSD offset, and DF for the MU calculation, with a reduction in human effort and linac beam-on time, it is recommended that Monte Carlo simulations be partially or completely integrated into the commissioning of electron are therapy.

A novel technique for post-mastectomy breast irradiation utilising non-coplanar intensity-modulated radiation therapy

The aim of this study was to investigate if non-coplanar intensity-modulated radiation therapy (IMRT) in the post-mastectomy setting can reduce the dose to normal structures and improve target coverage. We compared this IMRT technique with a standard partial wide tangential (PWT) plan and a five-field (5F) photon-electron plan. 10 patients who underwent left-sided mastectomy were planned to 50.4 Gy using either (1) PWT to cover the internal mammary (IM) nodes and supraclavicular fields, (2) 5F comprising standard tangents, supraclavicular fields and an electron field for the IM nodes or (3) IMRT. The planning target volume (PTV) included the left chest wall, supraclavicular, axillary and IM lymph nodes. No beams were directed at the right lung, right breast or heart. Mean dose-volume histograms were constructed by combining the dose-volume histogram data from all 10 patients. The mean PTV to receive 95% of the dose (V95%) was improved with the IMRT plan to 94.2% from 91.4% (p = 0.04) with the PWT plan and from 87.7% (p = 0.012) with the 5F plan. The mean V110% of the PTV was improved to 3.6% for the IMRT plan from 16.8% (p = 0.038) for the PWT plan and from 51.8% (p = 0.001) for the 5F plan. The mean fraction volume receiving 30 Gy (v30Gy) of the heart was improved with the IMRT plan to 2.3% from 7.5% (p = 0.01) for the PWT plan and 4.9% (p = 0.02) for the 5F plan. In conclusion, non-coplanar IMRT results in improved coverage of the PTV and a lower heart dose when compared with a 5F or PWT plan.

Application of the electron pencil beam redefinition algorithm to electron arc therapy

This project investigated the potential of summing fixed-beam dose distributions calculated using the pencil-beam redefinition algorithm (PBRA) at small angular steps (1 degree) to model an electron arc therapy beam. The PRBA, previously modified to model skin collimation, was modified further by incorporating two correction factors. One correction factor that is energy, SSD (source-to-surface distance), and field-width dependent constrained the calculated dose output to be the same as the measured dose output for fixed-beam geometries within the range of field widths and SSDs encountered in arc therapy. Another correction factor (single field-width correction factor for each energy) compensated for large-angle scattering not being modeled, allowing a more accurate calculation of dose output at mid arc. The PBRA was commissioned to accurately calculate dose in a water phantom for fixed-beam geometries typical of electron arc therapy. Calculated central-axis depth doses agreed with measured doses to within 2% in the low-dose gradient regions and within 1-mm in the high-dose gradient regions. Off-axis doses agreed to within 2 mm in the high-dose gradient regions and within 3% in the low-dose gradient regions. Arced-beam calculations of dose output and depth dose at mid arc were evaluated by comparing to data measured using two cylindrical water phantoms with radii of 12 and 15 cm at 10 and 15 MeV. Dose output was measured for all combinations of phantom radii of curvature, collimator widths (4, 5, and 6 cm), and arc angles (0 degrees, 20 degrees, 40 degrees, 60 degrees, 80 degrees, and 90 degrees) for both beam energies. Results showed the calculated mid-arc dose output to agree within 2% of measurement for all combinations. For a 90 degree arc angle and 5 x 20 cm2 field size, the calculated mid-arc depth dose in the low-dose gradient region agreed to within 2% of measurement for all depths at 10 MeV and for depths greater than depth of dose maximum R100 at 15 MeV. For depths in the buildup region at 15 MeV the calculations overestimated the measured dose by as much as 3.4%. Mid-arc depth dose in the high-dose gradient region agreed to within 2.2 mm of measured dose. Calculated two-dimensional relative dose distributions in the plane of rotation were compared to dose measurements using film in a cylindrical polystyrene phantom for a 90 degree arc angle and field widths of 4, 5, and 6 cm at 10 and 15 MeV. Results showed that off-axis dose at the ends of arc (without skin collimation) agreed to within 2% in the low-dose gradient region and to within 1.2 mm in the high-dose gradient region. This work showed that the accuracy of the PBRA arced-beam dose model met the criteria specified by Van Dyk et al. [Int. J. Radiat. Oncol. Biol. Phys. 26, 261-273 (1993)] with the exception of the buildup region of the 15 MeV beam. Based on the present results, results of a previous study showing acceptable accuracy in the presence of skin collimation, and results of a previous study showing acceptable accuracy in the presence of internal heterogeneities, it is concluded that the PBRA arced-beam dose model should be adequate for planning electron arc therapy.

An improved internal mammary irradiation technique in radiation treatment of locally advanced breast cancers

The purpose of the present study was to compare a new internal mammary irradiation technique with traditional techniques for locally advanced breast cancers in terms of sparing ipsilateral lung and heart and reducing the “cold” and “hot spots” in breast tissue. The new technique uses wide tangential fields for the first eight fractions of treatment. A medial internal mammary field (IMF) of electrons matched with narrowed tangential fields is used for the remaining fractions. Intensity‐modulated radiation therapy (IMRT) by means of segmented multileaf collimation (SMLC) is used in the narrowed tangential fields to improve the match between the electron and the photon fields. Treatment planning was performed to compare this technique to a wide‐tangential‐only technique and to a traditional oblique IMF technique for three patients with differing habitus. Film dosimetry was performed in a solid water phantom to confirm the planning results. For all three patients, the mean doses of the ipsilateral lung and the heart were significantly reduced with the new technique. The lung and the heart volumes were remarkably reduced at low‐dose levels (≤12GY) compared to the traditional IMF technique, and significantly reduced at all dose levels compared to the wide tangential technique. The new technique also reduced the “cold” and “hot spots” along the match plane between the IMF and the tangential fields compared to the traditional IMF technique. In conclusion, the new IMF technique shows dosimetric improvement compared to the traditional IMF technique in terms of the critical organ sparing and target dose uniformity.

PACS number: 87.53.Tf

Local???Regional Radiation Therapy After Breast Reconstruction: What Is the Appropriate Target Volume?

The oncologic safety and cosmetic outcome of immediate breast reconstruction in breast cancer patients requiring radiation therapy remains ill-defined. Between 1980 and 1998, 18 patients were treated at the University of Utah Medical Center with mastectomy, immediate breast reconstruction, and adjuvant radiation therapy delivered via an electron arc technique. A case-control study was performed matching reconstructed patients in a 1:2 ratio with patients undergoing mastectomy without reconstruction, using number of lymph nodes and tumor size. Median follow-up was 61 months for the reconstructed group. Five-year local-regional control, disease-free survival, and overall survival rates were 87%, 58%, and 74% respectively in the reconstructed group, versus 88%, 57%, and 67% respectively in the matched control group. Cosmesis was good/excellent in 11 of 13 living patients (85%). Significant capsular contraction occurred in 18% of prosthetic reconstruction patients, and revisional surgery was required in 24% of prosthetic reconstruction patients. Utilizing the electron arc technique, the median radiation dose to the chest wall at the midlevel of the ribs was 20% of the prescribed dose, and no patient failed deep to the implant. These results suggest that in appropriately selected patients, structures deep to the reconstruction are not at high risk for local-regional recurrence, and immediate breast reconstruction yields comparable local-regional control, disease-free survival, and overall survival rates to nonreconstructed patients, with acceptable cosmetic results.

Electron arc irradiation of the postmastectomy chest wall with CT treatment planning: 20-year experience

PURPOSE

Since 1980, electron arc irradiation of the postmastectomy chest wall has been the preferred radiotherapy technique at the University of Utah for patients with advanced breast cancer. We report the results of this technique in 156 consecutive Stage IIA-IIIB patients treated from 1980 to 1998.

METHODS

CT treatment planning was used in all patients to identify chest wall thickness and internal mammary lymph node depth. Computerized dosimetry was used to deliver total doses of 50 Gy in 5-1/2 weeks to the chest wall and the internal mammary lymph nodes with electron arc therapy. Patients were assessed for local, regional, and distant control of disease and for survival. Univariate and multivariate proportional hazards were modeled using a hierarchical nonproportional semiparametric model testing the following prognostic factors: age, stage, tumor size, number of positive lymph nodes, estrogen receptor status, and dose. End points evaluated included disease-free survival, cause-specific survival, and overall survival.

RESULTS

Eighty-one percent of patients were at high risk for local-regional failure because of > T2 primary tumor or > 3 positive axillary lymph nodes. The median number of positive lymph nodes was 5, and the median tumor size was 3.5 cm. Actuarial 10-year local-regional control and overall survival were 95% and 52%, respectively. In multivariate analysis, the only factor prognostic for disease-free survival, cause-specific survival, and overall survival was the number of positive lymph nodes (p < 0.001). The 10-year rates of local-regional control for patients with 0, 1-3, 4-9, and > or = 10 involved lymph nodes were 100%, 98%, 93%, and 89%, respectively. The only rates of acute and chronic radiotherapy toxicity > or = 2 by RTOG/EORTC criteria were skin related and observed in 44% and 10% for acute and late reactions, respectively.

CONCLUSION

These data demonstrate excellent local-regional control rates with electron arc therapy of the postmastectomy chest wall in patients with advanced breast cancer. Our 20-year experience with electron arc radiotherapy has demonstrated the safety and efficacy of this technique. The advantage of this technique is that the internal mammary lymph node chain can be easily encompassed while the dose to heart and lung is minimized; it also obviates match lines in areas of high risk.

The influence of patient geometry on the selection of chest wall irradiation techniques in post-mastectomy breast cancer patients

BACKGROUND AND PURPOSE

To evaluate three chest wall (CW) irradiation techniques: wide tangential photon beams, direct appositional electron field and electron arc therapy with regards to target coverage and normal tissue tolerance.

MATERIALS AND METHODS

Thirty-two post-mastectomy breast cancer patients were planned using three CW irradiation techniques. Computed tomography (CT) simulation was done on all patients and clinical target, heart and lung volumes were contoured. For each technique, dose distributions and dose-volume histograms (DVH) were calculated. Pass/fail criteria were applied based on volumetric target and critical structure dose coverage. Passing criteria for target was 95% of target receiving 95% of dose using a standard dose of 50 Gy/25 fractions, for heart </=10% volume receiving 50% dose (i.e. 25 Gy) and for lung </=25% volume ipsilateral lung receiving 50% dose (i.e. 25 Gy).

RESULTS

The number of patients optimally treated by each technique were as follows: wide tangential photon beams 23/32 (72%), direct appositional electron field 1/32 (3%), electron arc 4/32 (12.5%) and in 4/32 (12.5%) no technique was optimal. Geometric predictors for technique suitability include CW thickness, medial to lateral CW curvature, uniformity of superior to inferior CW curvature and length of mastectomy scar.

CONCLUSIONS

This study confirms the utility of CT planning and DVH analysis for treatment planning of breast cancer. Patient factors that predict for treatment technique suitability and aid in technique selection can be identified. In a small subset of patients, none of the currently studied techniques were optimal and more novel methods of chest wall irradiation are required.

Electron arc irradiation of the postmastectomy chest wall: clinical results

BACKGROUND AND PURPOSE

Since 1980 electron arc irradiation of the postmastectomy chest wall has been the preferred technique for patients with advanced breast cancer at our institution. Here we report the results of this technique in 140 consecutive patients treated from 1980 to 1993.

MATERIALS AND METHODS

Thoracic computerized tomography was used to determine internal mammary lymph node depth and chest wall thickness, and for computerized dosimetry calculations. Total doses of 45-50 Gy in 5 to 5 1/2 weeks were delivered to the chest wall and internal mammary lymph nodes via electron arc and, in most cases, supraclavicular and axillary nodes were treated with a matching photon field. Patients were assessed for acute and late radiation changes, local and distant control of disease, and survival. Patients had a minimum follow-up of 1 year after completion of radiation treatment, and a mean follow up interval of 49 months and a median of 33 months. All patients had advanced disease: T stages 1, 2, 3, and 4 represented 21%, 39%, 21% and 19% of the study population, with a mean number of positive axillary lymph nodes of 6.5 (range, 0-29). Analysis was performed according to adjuvant status (no residual disease, n = 90), residual disease (positive margin, n = 15, and primary radiation, n = 2), or recurrent disease (n = 33).

RESULTS

Acute radiation reactions were generally mild and self limiting. A total of 26% of patients developed moist desquamation, and 32% had brisk erythema. Actuarial 5 year local-regional control, freedom from distant failure, and cause-specific survival was 91%, 64%, and 75% in the adjuvant group; 84%, 50%, and 53% in the residual disease group; and 63%, 34%, and 32% in the recurrent disease group, respectively. In univariate Cox regressions, the number of positive lymph nodes was predictive for local failure in the adjuvant group (P = 0.037). Chronic complications were minimal with 11% of patients having arm edema, 17% hyperpigmentation, and 13% telangectasia formation.

CONCLUSION

These data demonstrate that local-regional control with electron are therapy of the postmastectomy chest wall is comparable to photon techniques. Acute radiation reactions are well tolerated and mostly of minor extent. A previous report demonstrated a significant reduction in the dose-volume relationship of the lung using the electron arc compared with two photon techniques. Consequently, with careful attention to treatment planning and dosimetry, electron arc therapy of the postmastectomy chest wall is safe and effective. The radiation dose to heart and lung is minimized without compromise on local control.

Design and production of wax compensators for electron treatments of the chest wall

The primary aim of electron treatment planning for the post mastectomy chest wall is to encompass the volume between the skin surface and the lung-rib interface while limiting dose to the lung. Electron energies for treatment of the chest wall are chosen based on the thickness of tissue between these two areas. Surgical defects or surface irregularities often result in differing thicknesses of tissue across the treatment volume, and patient-specific compensation is necessary to achieve the desired dose distribution. This is true whether the treatment plan is designed using fixed or rotational electrons to treat the chest wall. These clinical requirements are often met using custom shaped wax of varying thickness which conforms to the chest surface. This paper will discuss the treatment planning process used to design these compensators, creation and use of an exact duplicate of the patient's chest wall to aid in the production of these compensators, the production process itself, and verification of the completed compensator.

Dosimetric considerations in treating mediastinal disease with mantle fields: characterization of the dose under mantle blocks

PURPOSE

While the rationale for using mantle fields is well understood and the prescription of these fields is straightforward, the underlying complexity of the dose distributions that result is not generally appreciated. This is especially true in the choice of lung block design, which affects the dose to both the target volume as well as to the normal lung tissue. The key to the design of optimal lung blocks is the physician's perception of the complex relationship between the geometric and dosimetric aspects of heavily modified fields, as well as how the physical and anatomical properties of the target volume and the shape of the patient's lungs relate to the images visualized on simulator films.

METHODS AND MATERIALS

Depth doses and cross-beam profiles of blocks ranging in width from 1 cm to 10 cm were taken using an automated beam scanning system. These data were then converted to "shadow fields." The results were compared to open fields of the same size using standard methodology.

RESULTS

Shadow fields behave quite similarly to small, open fields in terms of x-ray-light field congruence, flatness, symmetry, and penumbra. There is a 2-3 mm rim between the edge of the block and the point at which it becomes nominally effective. The dose at the center of a block, which gives the normalization of the shadow fields, is given by a block transmission factor (BTF), which produces results in excellent agreement with measurements over a wide variety of block sizes and tissue depths.

CONCLUSION

The radiation dose under shielding blocks can be considerably higher than expected, and care must be exercised when drawing blocks close to critical structures. The effects of blocks can be described in terms of normalized shadow fields, which behave similar to narrow, open fields, but with a divergence characteristic of their position relative to the radiation source. The normalization value for these fields, which gives the relative dose under the block, can be obtained from a straightforward analytical expression, the BTF.

Improved field edge definition in electron arc therapy with dynamic collimation techniques

PURPOSE

The diffuse shape of the electron beam used for arc therapy requires collimation on the patient's surface to sharply define treatment field edges. The electron beam arcs 10-15 degrees past field edges defined by a custom fitted and manufactured cast which acts as a tertiary collimator. This allows the entire beam profile to be integrated at the field edge. The tertiary collimator is heavy and bulky, requiring two therapists to lift and position the cast. An alternative technique for field edge definition would require the electron arc collimators to dynamically close to zero, while maintaining the projection of the leading edge of the field coincident with the geometric edge of the treatment field. This would allow integration of the entire electron arc profile and maintain a sharply defined treatment edge at the medial and lateral margins of the arc. The present customized cast could be replaced by generic lead strips at only the superior and inferior treatment field borders. This study investigates the dosimetry of dynamically collimated electron arc treatment volumes at field margins and its potential for eliminating the need for tertiary collimation at the arc field margins.

METHODS AND MATERIALS

Electron arc isodose distributions were calculated using a pencil beam algorithm for treatment volumes defined by tertiary collimation at the surface of a cylindrical phantom and compared to distributions generated by simulating dynamic collimation to define the same field edges. Phantom measurements were performed using film densitometry to verify computer predictions.

RESULTS

Penumbra width is one measure of the sharpness of dose fall off at a treatment field edge. We define it as the distance between the 90% and 20% isodose lines at the field edge measured orthogonal to the incident electron beam. Calculations and phantom film densitometry measurements were performed for electron energies from 6-20 MeV. Dynamic and tertiary collimation both reduce penumbra width by approximately 50% compared to no collimation. There is a small advantage in minimizing penumbra width at low electron energy with tertiary collimation. This shifts to a small advantage with dynamic collimation at high electron energy.

CONCLUSION

Dynamic collimation produces a field edge isodose distribution equivalent to tertiary collimation for clinical purposes. These results suggest that tertiary collimation at medial and lateral electron are treatment field margins can be eliminated with dynamic collimation. This should result in greater clinical acceptance of breast electron arc therapy as the capacity for dynamic collimation is added to the next generation of linear accelerators.

Expanding the use and effectiveness of dose-volume histograms for 3-D treatment planning. I: Integration of 3-D dose-display

PURPOSE

A technique is presented for overcoming a major deficiency of histogram analysis in three-dimensional (3-D) radiotherapy treatment planning; the lack of spatial information.

METHODS AND MATERIALS

In this technique, histogram data and anatomic images are displayed in a side-by-side fashion. The histogram curve is used as a guide to interactively probe the nature of the corresponding 3-D dose distribution. Regions of dose that contribute to a specific dose bin or range of bins are interactively highlighted on the anatomic display as a window-style cursor is positioned along the dose-axis of the histogram display. This dose range highlighting can be applied to two-dimensional (2-D) images and to 3-D views which contain anatomic surfaces, multimodality image data, and representations of radiation beams and beam modifiers. Additionally, as a range of histogram bins is specified, dose and volume statistics for the range are continually updated and displayed.

RESULTS

The implementation of these techniques is presented and their use illustrated for a nonaxial three field treatment of a hepatic tumor.

CONCLUSION

By integrating displays of 3-D doses and the corresponding histogram data, it is possible to recover the positional information inherently lost in the calculation of a histogram. Important questions such as the size and location of hot spots in normal tissues and cold spots within target volumes can be more easily uncovered, making the iterative improvement of treatment plans more efficient.

Electron arc therapy of the postmastectomy prosthetic breast

PURPOSE

Reconstructive surgery of the postmastectomy breast presents new challenges to postoperative radiation therapy. This paper evaluates the dosimetric significance of oblique incidence of the electron field on the reconstructed mound during arc therapy.

METHODS AND MATERIALS

Using film densitometry, the relative dose distributions resulting from electron arc therapy incident on phantoms simulating the breast and chest wall dimensions of an actual patient are evaluated.

RESULTS

Irradiation of the breast phantom in normal supine position results in constriction of the dose distribution at the junction of the reconstructed mound with the chest wall. Angulation of the phantom to provide a more normal incidence of the electron beam during arc reduces the constriction by minimizing obliquity of the incident electrons.

CONCLUSION

These measurements suggest that, with proper positioning of the patient relative to the incident electron beam, electron arc therapy may be used as an alternative treatment technique for treatment of the postmastectomy reconstructed breast and chest wall.